You also end up with this nice bit of inconsistency:
>>> x = myint(7)
>>> from operator import index
>>> range(10)[6:x]
range(6, 7)
>>> range(10)[6:x.__index__()]
range(6, 8)
>>> range(10)[6:index(x)]
range(6, 7)
>>>
Granted, it's insane to have __index__() return a different value like this, but in my specific use case, it's the type of object returned from operator.index() that's the problem. operator.index() returns the subclass instance while obj.__index__() returns the int.
(The use case is the IntEnum of PEP 435.)